1. Field of Invention
The present invention relates generally to toners and toner processes, and more specifically, relates to emulsion aggregation toner compositions containing wax having a reduction of wax protrusions and coarse particles.
2. Description of Related Art
The art of preparing an emulsion aggregation (EA) type toner is known in the art and forms toners by aggregating a colorant with a latex polymer formed by batch or semi-continuous emulsion polymerization. For example, U.S. Pat. No. 5,853,943 (hereinafter xe2x80x9cthe ""943 patentxe2x80x9d), which is herein Incorporated by reference, is directed to a semi-continuous emulsion polymerization process for preparing a latex by first forming a seed polymer. In particular, the ""943 patent describes a process comprising:
(i) conducting a pre-reaction monomer emulsification which comprises emulsification of the polymerization reagents of monomers, chain transfer agent, a disulfonate surfactant or surfactants, and optionally, but preferably, an initiator, wherein the emulsification is accomplished at a low temperature of, for example, from about 5xc2x0 C. to about 40xc2x0 C.:
(ii) preparing a seed particle latex by aqueous emulsion polymerization of a mixture comprised of (a) part of the monomer emulsion, from about 0.5 to about 50 percent by weight, and preferably from about 3 to about 25 percent by weight, of the monomer emulsion prepared in (i), and (b) a free radical Initiator, from about 0.5 to about 100 percent by weight, and preferably from about 3 to about 100 percent by weight, of the total initiator used to prepare the latex polymer at a temperature of from about 35xc2x0 C. to about 125xc2x0 C., wherein the reaction of the free radical initiator and monomer produces the seed latex comprised of latex resin wherein the particles are stabilized by surfactants;
(iii) heating and feed adding to the formed seed particles the remaining monomer emulsion, from about 50 to about 99.5 percent by weight, and preferably from about 75 to about 97 percent by weight, of the monomer emulsion prepared In (ii), and optionally a free radical initiator, from about 0 to about 99.5 percent by weight, and preferably from about 0 to about 97 percent by weight, of the total Initiator used to prepare the latex polymer at a temperature from about 35xc2x0 C. to about 125xc2x0 C.; and
(iv) retaining the above contents In the reactor at a temperature of from about 35xc2x0 C. to about 125xc2x0 C. for an effective time period to form the latex polymer, for example from about 0.5 to about 8 hours, and preferably from about 1.5 to about 6 hours, followed by cooling.
Other examples of emulsion/aggregation/coalescing processes for the preparation of toners are illustrated in U.S. patents, the disclosures of which are totally incorporated herein by reference, such as U.S. Pat. No. 5,290,654, U.S. Pat. No. 5,278,020, U.S. Pat. No. 5,308,734, U.S. Pat. No. 5,370,963, U.S. Pat. No. 5,344,738, U.S. Pat. No. 5,403,693, U.S. Pat. No. 5,418,108, U.S. Pat. No. 5,364,729, and U.S. Pat. No. 5,346,797. Also of interest may be U.S. Pat. No. 5,348,832, U.S. Pat. No. 5,405,728, U.S. Pat. No. 5,366,841, U.S. Pat. Nos. 5,496,676, 5,527,658, U.S. Pat. No. 5,585,215, U.S. Pat. No. 5,650,255, U.S. Pat. No. 5,650,256 and U.S. Pat. No. 5,501,935.
In addition, examples of wax containing toners, the disclosures of which are incorporated herein by reference in their entireties, include U.S. Pat. No. 5,482,812, U.S. Pat. No. 5,688,625, U.S. Pat. No. 5,994,020, U.S. Pat. No. 6,210,853 and U.S. Pat. No. 6,294,606.
For toner compositions with low gloss and high transmission optical density for low oil or oil-less fuser applications, it is preferable to incorporate high levels of colorant, wax and crosslinked latex resin. Such toners prepared generally result in particles with excessive wax protrusions at the toner surface which result in poor particle size, poor particle size distribution control, and unstable triboelectric charging performance.
The said wax protrusions tend to develop on the surface of the toner, therein producing coarse particles, during a cooling stage wherein the toner slurry is cooled to an appropriate working temperature of about 25xc2x0 C. However, the wax is advantageously included in the particles because it aids in fusing release.
Additive loading can also be performed to mask wax protrusions on the toner. However, additive loading may also lead to the development of coarse particles that must then be separated from the good toner particles. Separating the coarse particles and additive loading increases the costs associated with the production of the toner.
The present invention embodies a means by which wax protrusions and coarse particles can be reduced in emulsion aggregation toners while still obtaining the advantageous aspects of the wax at a reasonable expense.
In one embodiment of the present invention, a wax having a higher molecular weight is introduced in the toner formulation process. A significant reduction in wax protrusions requires that a wax be chosen that has a melt distribution substantially above the coalescence temperature of the toner. A higher molecular weight wax causes the melt distribution of the system to shift towards a higher temperature.
In another embodiment, waxes of different molecular weights are introduced into the toner formulation process. In a preferred approach, the two or more waxes are melt homogenized. The resulting aqueous wax dispersion is then incorporated into the toner formulation process.
In another preferred approach, two or more wax dispersions are melt mixed together before homogenization. The resulting aqueous wax dispersion is then incorporated into the toner formulation process.